fix(coverage): missing files in the coverage report if they have no tests

This ensures that un-executed files (i.e. files that aren't tested) are
included in the coverage report. The current behavior is that
coverage.py excludes them by default.

See https://coverage.readthedocs.io/en/7.6.10/source.html#execution

Author: BurnzZ

CHANGELOG.md

@@ -84,6 +84,7 @@ Unreleased changes template.
   {obj}`--venvs_use_declare_symlink=no` to have it not create symlinks at
   build time (they will be created at runtime instead).
   (Fixes [#2489](https://github.com/bazelbuild/rules_python/issues/2489))
+* (coverage) Fix missing files in the coverage report if they have no tests.
 
 {#v0-0-0-added}
 ### Added

python/private/python_bootstrap_template.txt

@@ -433,6 +433,14 @@ def _RunForCoverage(python_program, main_filename, args, env,
 relative_files = True
 ''')
   PrintVerboseCoverage('Coverage entrypoint:', coverage_entrypoint)
+
+  instrumented_files = [abs_path for abs_path, _ in InstrumentedFilePaths()]
+  unique_dirs = {os.path.dirname(file) for file in instrumented_files}
+  source = ",".join(unique_dirs)
+
+  PrintVerboseCoverage("[coveragepy] Instrumented Files:\n" + "\n".join(instrumented_files))
+  PrintVerboseCoverage("[coveragepy] Sources:\n" + "\n".join(unique_dirs))
+
   # First run the target Python file via coveragepy to create a .coverage
   # database file, from which we can later export lcov.
   ret_code = subprocess.call(
@@ -443,6 +451,7 @@ relative_files = True
       "--rcfile=" + rcfile_name,
       "--append",
       "--branch",
+      "--source=" + source,
       main_filename
     ] + args,
     env=env,